net: rds: changing the return type from int to void

as result of function rds_iw_flush_mr_pool is nowhere checked,
changing its return type from int to void.
also removing the unused variable rc as there is nothing to return

Signed-off-by: Saurabh Sengar <saurabh.truth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>